Format GB cell numbers as 07xxx xxxxxx

[fiznool]

As mentioned in #123, GB mobile numbers are not currently valid.

This PR changes the generator to create numbers in the format 07xxx xxxxxx.

[codecov-io]

Codecov Report

Merging #144 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
- Coverage      94%   93.96%   -0.05%     
==========================================
  Files          94       94              
  Lines        2253     2253              
  Branches      317      317              
==========================================
- Hits         2118     2117       -1     
- Misses        135      136       +1

Impacted Files	Coverage Δ
api/1.1/data/FI/inject.js	91.66% <0%> (-4.17%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e0e221b...5450413. Read the comment docs.